There are several driving mechanisms of an actuator, such as a ball screw/ball nut mechanism. The ball screw/ball nut mechanism comprises a ball screw, and a ball nut being engaged with the ball screw. The rotative movement of the ball nut has been prohibited, and when the ball screw is rotated by a servo motor, the ball nut will move axially in the forward and the rearward directions. Accordingly, the ball nut will accept various devices to be attached thereto.
The ball screws and the ball nuts used for the above mechanism were firstly manufactured by respective special machining apparatus, and then, the finishing of the thread surfaces, etc. were done by a lapping operation. For example, when the lapping is applied to the inner thread surface of a ball nut, a lapping tool, which is generally called as “lapping bar”, is used. The lapping bar has a thread part formed on the outer surface of an axial member, and also has slits which have been formed in the axial direction.
When the lapping operation is done, first, the lapping tool is rotated. Then the ball nut, i.e. the object to be lapped, becomes engaged with the lapping tool, whereby the lapping is applied to the inner thread part of the ball nut. At that time, the lapping tool is elastically displaced according to the elastic displacement range of the slits, and the lapping operation is done by means of the elastic force of the lapping tool, which has been accumulated by such a displacement of the slits.
However, the conventional lapping tools discussed as above have the following problems.
First, as discussed above, the lapping tool has the elongating slits formed in the axial direction, and the slits serve the substantial role for the lapping operation. However, because the tip part of the slits is formed to be open, this will cause the warping of the lapping tool, whereby the lapping accuracy will be deteriorated, and consequently the life of the lapping tool will be shortened.
Second, because the lapping diameter of the conventional lapping tool has been fixed, when the lapping is applied to ball nuts having various diameters, the lapping must be applied by various lapping tools having the corresponding various diameters, thus it is necessary to prepare and change various diameters of lapping tools in advance.
In the light of technical backgrounds and problems according to the conventional lapping tools discussed as above, it is an object of the present invention to provide a lapping tool, having the improved lapping accuracy and the long tool life. Further, it is another object of the present invention to provide a lapping tool of which lapping diameter is adjustable, corresponding to various lapping diameters.
The present invention is relevant to several prior inventions, such as Japanese Unexamined Patent Publication No. 2000-343327. This prior invention relates to a reamer, wherein the adjustment and setting of the finish diameter by reamer can be done mechanically.